Remove unused variable.
Expose first-class values for ML types used to view primitive types.
Coercions between identical primitive types.

This is useful for exposing the representation of abstract types:

  # coerce clock_t int64_t;;
  - : clock_t -> int64 = <fun>
Simplify pointer coercion.
Merge pull request #95 from yallop/primitive-identity-coercions

Primitive identity coercions